Since exposure to carbon dioxide partial pressures exceeding about 7.6 mm Hg (partial pressure of about 1.0%) for extended periods of time can cause adverse health problems in humans and other mammals, and since mammals biologically produce carbon dioxide, closed environmental systems such as submarines and spacecraft, often require the removal of this carbon dioxide and carbon dioxide produced by other processes. Various removal techniques have been developed using sorbents which remove carbon dioxide and/or water from a gaseous stream by physical or chemical means. These sorbents include supported and unsupported regenerable and nonregenerable carbon dioxide sorbents such as metal oxides, molecular sieves, alkali metal carbonates, alkali metal hydroxides, and combinations thereof, among others.
Important characteristics of these sorbents, especially those utilized in closed environment systems, include regenerability, structural integrity, absorption-desorption cyclical life, energy requirements for regeneration, and volume and weight considerations. Since metal oxide-alkali metal carbonate sorbents can be formed without inert material yet retain structural integrity while possessing an extended cyclical life and high carbon dioxide loadings, they are useful for carbon dioxide removal in closed environments. Some of these metal oxide-alkali metal carbonate sorbents are regenerable and unsupported, having cyclical lives up to or greater than about 100 cycles and carbon dioxide loadings of up to about 20 lb/ft.sup.3.
In the closed environment provided by a submarine, amines, such as monoethanolamine and diethanolamine, are often used to absorb carbon dioxide. These amines are utilized in an aqueous phase, typically 25 to 30 weight percent (wt %) amine. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced to the bottom of the tower. The amine intimately contacts the gaseous stream in a countercurrent fashion and chemically absorbs carbon dioxide in the gaseous stream. Desorption of this carbon dioxide is then accomplished in a thermal regeneration process at temperatures in excess of 150.degree. F. During desorption, carbon dioxide and water vapor evolve and are then separated by condensing the water vapor in a heat exchanger. Since the gaseous stream often contains acid gas contaminants which limit the amine based sorbent life through irreversible reaction, the amine sorbent solutions typically possess a life of up to six months of continuous use.
Although the above regenerable sorbents provide useful means for carbon dioxide and water sorption, artisans perpetually seek improved sorbents requiring lower volume and weight penalties, lower energy requirements for regeneration, and possessing high cyclical life and continuous operation. Therefore, what is needed in the art is a carbon dioxide and water sorbent capable of continuous operation at high carbon dioxide removal rates with low volume, weight, and power requirements.